Method of preparing bar stock in the manufacture of piezoelectric quartz crystals



Jam 14, 1947. 'R H, BRQSEKER AL 2,414,087

METHOD 0F PRBPARING BAR STOCK IN THE HANUFACTURE 0F PIEZOBLECTRIC QUARTZ CRYSPALS Filed Aug. 10, 1942 Fig@ Z xl 1 Q f5 Q 3 MscHA/vyg; fm g l 6 5J' ammiw FW 3 l l u o I "j 2 F319@ "5 mvENToRs 5 m.' I l l ROLAND'MBRosEA?. @ya 4V BY AME .HESENAU R lies along the Y or mechanical axis.

staats as. 1i, ist? METHOD F PREPARING ABAR STOCK m THE vMIANUFACTURE QUARTZ CRYSTALS Roland H. Broseker, Anne Arundel County, and James L. Hessenauer, Baltimore, Md., assignors to Bendix Aviation Corporation, South Bend, Ind., a corporation of Delaware Application August 1o, i942, serial No. 454,284 9 claims. (C1. 1v1-:i275v This invention relates 'to the preparation of -bar stock in the manufacture of piezo-electric quartz crystals and more particularly to an 0D- tical method for determining the orientation of guide lines for the cutting of vbar stock from a. crystal slab. l

In the past, the above determination has been made by employing a number of different pieces of test apparatus. Upon receiving the rough quartz crystals or aggregations ofcrystals, the position ofthe optical axis is determined either by well known optical methods or by the utilization of one of the undamaged faces of the crystal as a reference, if such a face exists. the difficulty of making the determination optically, many establishments purchase only quartz crystals having at least one intact face and the entire crystal is cut up using this face as a reference. The cutting planes utilized are normally parallel to the basal plane and consequently are perpendicular to the optic or Z axis. The individual pieces thus obtained -from each crystal are then kept together during the subsequent operations in order that the use of the reference face not be lost. Next, the slabs thus obtained are examined in polarized light to locate regions y in which optical twinning is present and to determine the right-handedne'ss or left-handedness of the crystal. For convenience in reference, the quality of right-handedness or left-handedness is sometimes termed the optical rotatory polarity. There are certain types of electrical twinning of the crystal which cannot be detected during these tests and as a, result some pieces are not rejected until much further along in the process of fabrication after a considerable amount of` work has been done on them. As the next step, the quartz is placed in a device for determining its polarity by means of the output voltage which is developed as a result of impact, and the crystal faces are marked accordingly. After obtaining this information, the slab is then cut up into rectangular bars using a cutting plane approximately normal to the first mentioned cutting plane.`

This plane may have any direction but usually The bar thusobtai-ned is then inserted in a cutting machine' and cut up into crystal blanks whose faces make the necessary angle with the Z axis to secure the desired temperature coeiilcient. For

Because of )F PIEZOELECTRIC the type of known commercially as AT. this angle is ap// ximately` 35 degrees, 22 minutes, while the BT cut is made at an approximate angle of 49 degrees, 47 minutes. In the bar obtained as above, the Z axis is perpendicular to one of the sides, and the cuts, therefore. are made necessarily atl a considerable angle to the major axis of the piece with the loss of much quartz.

It is an object of this invention to .provide a method for the easy and rapid determination of guide lines for thecutting of bar stock in the manufactureof piezo-electric quartz crystals.

It is a further object of this lnventionto pro` vide such a method which makes more eicient use ofthe .crystal stock by eliminating waste inherent in previously known methods of determining guide lines and cutting bar stnck from a cry-stal slab.

, trical polarity of the piece, which disclose whether Figure 2 is a, diagram showing the preparatory i 'cut used on the native crystal.

Figure 3 is a. section diagram illustrating the apparatus used in producing the desired images.

Figure .4 is an illustration of the pattern seen With a negative crystal face adjacent the observer in the apparatus of Figure 3.

`:Figure 5 is an illustration of the pattern seen with a positive crystal face adjacent the observerV in the apparatus of Figure 3.

Figure 6 is an illustration showing the manner to the mechanical axis of the crystal,

The

optical axis 2 is located by using one of the crystal faces as a reference plane or by examining the crystal in convergent polarized light. The

crystal I is now molded into a block of plaster cf paris in such a manner -that the optical axis bears a predetermined relation to one of the surfaces of said block,l this block serving as a jig to securethe crystal I while it is sawed into the slabs 3 of Figure 2 in a suitable machine. It will be noted'from Figure 2 that the cutting planes are substantially parallel to the optical axis`2 and Upon the completion of the sawing operation, the slabs 3 are removed from the plaster of paris matrix and,4 when placed close together would have somewhat the form shown in Figure 2, whence the manner in which they were cut out of the crystal I is obvious. In preparation for the next step' in the examination of .the slabs 3, they are immersed 'in a suitable etching solution such as a 48% solution of commercial hydroiluoric acid for a period of three to four hours. At the completion of the etching process, they are removed from the solution and thoroughly washed and dried. The complete optical test set up is shown in Figure 3 in which 4 is an opaque rectangular enclosure secured to the top of .the testing table 5 by the screws 6. Located within this enclosure 4 is a suitable light source lsuch as the ordinary incandescent bulb 1 mounted in the socket 8. A

pinhole 8 is pierced through the upper surface of l the enclosure 4 directly above the light source 'I. The specimen slab 3 to be tested is placed on .the observation table formed by the upper surface o! the enclosure where' it is traversed by the light emanating from the pinhole 9, and the observer then inspects the-patterns seen on-the emergent surface of the slab 3, that is, the side nearest the ifobserver, to determine the desired characteristics and the direction of cut for the preparation oi bar stock from the slab. It will be noted that in this test, the Z axisis parallel to the surface of the testing table and the light from the pinhole therefore traverses` the slab in a direction substantially perpendicular to that axis. A structure producing excellent results has been made by using .062 inch brass sheet for the enclosure 4 with a .013 inch circular opening for the pinhole 9.

The iigures seen with the negative face of the crystal slab 3 adjacent the observer in the apparatus of Figure 3 are shown in Figure 4 in which the direction of the optical axis is indicated by the line 2. When a piece of left-handed quartz is examined, the solid line parallelogram I0 is seen, in which the short side is parallel to the optical axis and the long side makes an angle of approximately 52 degrees with this axis. The sides of this figure appear as bright lines against a gray or dark background. Since an AT cut .crystal blank makes an angle of 35 degrees, 22

minor axis of the bar rather than 35 degrees and a considerable increase in the number of crystal blanks which may be obtained from a given piece of quartz is thereby effected due to the decrease in the amount of waste at theends. A similar method may be used in the'preparation of bars for the manufacture of BT cut crystal blanks. BT cut crystal blanks lie in aplane making an angle of 49 degrees, 47 minutes with the optical axis and the guide lines for the cutting of bar stock are therefore laid out substantially perpendicular to the long side of the parallelogram described above.

When a piece of right-handed quartz is. ex-

amined, the dashed line parallelogram II of Figure 4 is seen and the AT bar stock in this case --is again cut parallel to the long sides of the .parallelogram The manner in which the lefthanded crystal is cut into bars is shown in Figure 6 in which the cutting planes I2 are clearly visible. With 'a positive face adjacent the observer, the double ended arrow I3 of Figure 5 is seen, the same image being present for both right and left-handed quartz. The arrow I3 lies perpendicular to the optical axis and along a mechanical axis of the crystal slab 3. In the present procedure, this arrow serves merely `to identifythe positive' face although if desired it may be us'ed as a reference to establish the cutting line.

When the above system is employed, it is no longer necessary to keep the groups of slabscut from a native crystal of quartz together so that the reference plane is not lost, as the polarity,

-right-handedness or left-handednss, and the direction in which the cut is to be made are de# termined within a few seconds using the above method and apparatus. As the output of the slab cutting machines can thus be thrownv into a common container and, after being etched, sent'to A A group of personnel by a factor of three while the above method 0f cutting the bar stock has also apprcciably increased the yield of crystal blanks from a given quantity of raw material.

There is a type of twinning termed electrical twinning which is notreadily detected by optical methods. In a slab exhibiting this defect, the same figure is seen on both sides of the slab, that is, either the double ended arrow characteristic of the positive face or the parallelogram characteristic of the negative face. As would be expected, this type of quartz is inactive in that the piezo-electric effect is absent due to cancellation of the two polarities. This test is made by simply turning the slab over on the observation table and watching for the image change noted in good quartz. Very often slabs are found in which the polarity is not the same at all points and it is then desirable to lay out the lines of cut for the bar stock so that only one polarity exists within a given bar. Defects of this type are generally referred to as optical twinning.

Using the apparatus of Figure 3, the lines of acm-,cav

A few experiments have served to establish the fact that the character of the image observed is 'determined by the character of the emergent face Whichis remote from the pinhole. In one fof these experiments a slab of quartz was prepared by polishing the positive face and etching the negative face. Placing this slab on the observation table of Figure 3 with the polished face adjacent the pinhole, the parallelogram characteristic of a negative face was seen by the observer. nUpon the inversion of the piece, thus placing the negative face adjacent the pinhole, merely the pinhole itself was seen, the characteristic image being absent.` However, as the piece was now lifted from the pinhole, the parallelogram indicating a negative face was once' more seen, and this imageincreased in size as the separation between the pinhole and the negative face was increased. The parallelogram they are produced by the action of regularly oriented crystalline facets on the light passing through them and that these facets are brought out from the structure of the slab by the action of the etching fiuid. This hypothesis has been found to satisfactorily explain all the observed phenomena and appears to explain satisfactorily the fact that the images described above are destroyed if the etching process continues for too long a time. Since the action taking place is apparently in no way connected with the double refraction associated with crystalline quartz, this system of axis determination may also be employed in conjunction with otherI crystalline materials after preparation of the samples in a manner equivalent to that above described.

It will be obvious that many changes and modications may be made in the invention without departing from the spirit thereof asv expressed in the foregoing description'and in the'appended claims.

We claim:

1 In the manufacture of AT cut piezo-electric quartz crystals the method of preparing bar stock which comprises. cutting a slab from the mother e above I patterns isnot precisely known, it is believed that crystal with a cutting plane substantially parallel to the Y and Z axes, etching said slab, placing said slab adjacent an illuminated pinhole with the positive face nearest the pinhole, marking said slab with a guide line substantially parallel to the long side of the parallelogram shaped light pattern seen in the negative face of said crystal slab, and cutting said slab along said guide line.

2. In the manufacture'l of .AT-cutpiezo-elec- `tric quartz crystals, the method of preparing bar stock which comprises, cutting a slab from the i mother crystal with a cutting plane substantially 6 l stock which comprises, cutting a slab from the mother crystal with a cutting plane parallel to the Y and Z axes, etching said slab, placing said slab adjacent an illuminated pinhole with the positive face nearest the pinhole, marking said slab with a guide line substantially perpendicular to the long sides of the parallelogram shaped light pattern seen in the negative face of said crystal slab, and cutting saidslab along saicl guide line.

4. In the manufacture of BT-cut piezo-electric quartz crystals, the method of preparing bar stock which comprises, cutting a slab from the mother crystal with a cutting plane parallel to the Y and Z axes, etching said slab, placing said slab adjacent an illuminated pinhole with the positive face.

nearest the pinhole, marking said slab with guide lines perpendicular to the long sides of the parallelogram shaped light pattern seen in the negative face of said crystal slab within 15 degrees, and cutting said slab along said guide lines.

5. In the production of wafer like piezo-electric quartz crystal blanks, the method of manufacture which comprises, cutting a slab from the mother crystal with a cutting plane substantially parallel to the Y and Z axes, etching said slab, placing said slab adjacent an illuminated pinhole with the positive face nearest the pinhole, niark--4 ing said slab with guide lines making an angle, which constitutes substantially an integral multiple of ninety degrees, with the long sides ofthe parallelogram shaped light pattern seen in the negative face of said crystal slab, cutting said slab into bars along-sa`id guide lines, and slicing said bars into wafers having their principal surfaces substantially perpendicular to said guide lines.

6. In the manufacture of AT cut piezo-electric quartz crystals the method of preparing crystal wafer blanks which comprises, cutting a slab from the mother crystal with a cutting plane substanshaped light pattern seen in the negative face of said crystal slab, cutting said slab along said guide lines and cutting the resulting bars into wafers having' their principal surfaces substantially perpendicular to said guide line. l

4'7. In the manufacture of AT-cut piezo-electric quartz crystals, the method of preparing crystal wafer blanks which comprises, cutting a slab from the mother crystal with a cutting plane substantially parallel to the Y and Z axes, etching said slab, placing said slab adjacent an illuminated pinhole with the positive face nearest the pinhole, marking said slab with guide lines paralleling the long sides of the `parallelpgram shaped light pattern seen in the negative face of said slab within 15 degrees, cutting said slab along said guide lines, and cutting the resulting bars into wafers having their principal surfaces substantially'perpendicular to said guide lines.

8. In the manufacture of piezo-electric quartz crystals,l the method of preparing bar stock which comprises, cutting a slab from the mother crystal with a cutting plane parallel to the Y and Z axes, etching said s lab, placing a cut face of said slab adjacent an' illuminatedpinhole, producing therel by a parallelogram shaped lightdispersion pattern in the emergent face of said slab, and utilizing a side of said parallelogram as a referencefor the marking of guide lines on said emergent face,

said guide lines' making an angle with said long v tive face nearest the pinhole, marking said slab with guide lines making an angle, which is substantially an integral multiple of ninety degrees,

. wlththe long sides of the parallelogram shaped light pattern seen in the negative face of said crystal slab, and cutting said slab along said guide lines.

ROLAND H. BROSEKER. JAMES L. HESSENAUER. 

